What happens if you put the rotor inside cap backwards a car?

Putting the rotor on a car's distributor cap backwards will prevent the engine from running. The rotor's job is to direct high voltage to the correct spark plug wire in the correct firing order. If it's backward, the spark will go to the wrong cylinder at the wrong time, causing misfires and preventing the engine from starting or running smoothly. In short, it completely disrupts the ignition system.